1. Field of the Invention
The present invention relates to an apparatus for and a method of construction of a spindle motor which provides for an accurate and precise rotation, and more particularly, to an apparatus for and a method of construction of a motor for a hard disk drive (HDD) of a magnetic disk device or for other information storage devices. The present invention also relates to an information storage device incorporating a novel spindle motor.
2. Description of the Related Art
FIG. 1 shows the construction according to the related art of a spindle motor having a hub, rotated by an electromagnetic motor, supported in a freely rotatable manner on a bearing through a rotary shaft. With this spindle motor, a rotary shaft 92 is supported in a freely rotatable manner on a bearing unit 91 which is fixed to a base 90. A hub 93 is fixed onto the upper-end protruding section of the rotary shaft 92, and the hub 93 is allowed to rotate with the rotary shaft 92 as a center by means of a motor comprising a stator unit 94 fixed to the base 90 and a magnet 95 fixed to the hub 93. A magnetic disk (not shown) is mounted on the hub 93 and which therefore rotates together with the hub 93.
The bearing unit 91 is constructed with a sleeve 91A which is subjected to the radial load of the rotary shaft 92, and with a counter plate 91B which is subjected to the thrust load of the rotary shaft 92. The counter plate 91B is fixed to the lower-end face of the sleeve 91A and closes the lower-end opening of a shaft hole 91a of the sleeve 91A, but the upper side of the shaft hole 91a is left open. On the other hand, the rotary shaft 92 has a flange 92A at the lower-end section thereof. A lubricant is supplied between the bearing unit 91 and the rotary shaft 92, and there is provided a dynamic pressure generating section between them by which to generate a dynamic pressure to the lubricant.
The dynamic pressure generating section can be divided into a radial dynamic pressure generating section, which is subjected to the radial load of the rotary shaft 92, and a thrust dynamic pressure generating section, which is subjected to the thrust load of the rotary shaft 92. The radial dynamic pressure generating section has a plurality of grooves formed in the peripheral direction along the inner peripheral face of the sleeve 91A. Whereas, the thrust dynamic pressure generating section has a plurality of grooves formed in the peripheral direction respectively on the upper face of the counter plate 91B facing the flange 92A and on the step section 91b of the sleeve 91A facing the flange 92A. Dynamic pressure is generated when the lubricant supplied to the grooves is pressurized due to the rotation of the rotary shaft 92. It then becomes possible to maintain the rotary shaft 92 in a non-contact state and to support it highly accurately during rotation at a low torque by the generation of the dynamic pressure. As such grooves V-shaped herring bone grooves, which converge in the direction of rotation of the rotary shaft 92, are generally used.
If two rows of the radial dynamic pressure generating section were installed in the axial direction of the rotary shaft 92, the rigidity of the bearing increases and the oscillating characteristics of the rotary shaft improve. Furthermore, the longer the span between the two rows, the more effective such effects are. However, because the rotary shaft 92 of the spindle motor shown in FIG. 1 has a flange 92A at the lower-end section thereof, the length of the shaft section of the lower side from the flange 92A becomes very short (even to the point of being non-existent). Accordingly, it has been difficult to install two rows of radial dynamic pressure generating sections. One may think of making the shaft section longer in order to install two rows of radial dynamic pressure generating sections, but such a measure cannot accommodate the recently required trend of making a spindle motor thinner. That is to say, with a conventional spindle motor, it is difficult to allow two rows of radial dynamic pressure generating sections to be installed to satisfy the requirement of making the motor thin, and because of this, the improvement in oscillating characteristics of the motor has been impeded.